1. Field of the Invention
A heat exchanger according to the present invention is to be utilized as a built-in condenser for an automobile air conditioner.
2. Description of the Prior Art
An automobile air conditioner has a built-in vapor compression refrigerator. In the vapor compression refrigerator, high-temperature coolant discharges from a compressor under high pressure passes through a condenser 1 illustrated in FIG. 2 and is condensed and liquefied. The condenser 1 as described in, e.g., Japanese Patent Publication No. Hei. 5-228620, is created by brazing members of aluminum alloy to each other in combination. First, a pair of cylindrical headers 2a, 2b closed at both ends are spaced a distance away from each other, and a plurality of flattened heat-transfer tubes 3, 3 are provided across and between the inner sides of the pair of headers 2a, 2b (i.e., between the side surfaces of the headers facing each other). These flattened heat-transfer tubes 3, 3 are spaced away from each other and are connected at one end to the header 2a and at the other end to the header 2b. Both ends of the flattened heat-transfer tubes 3, 3 are respectively brazed to the headers 2a, 2b, so that the joints are air and fluid tight. Corrugated fins 4, 4 made by corrugating strip-shaped thin plates of aluminum alloys are sandwiched between the adjacent flattened heat-transfer tubes 3, 3, thereby constituting a core 5.
To condense and liquefy the high-temperature coolant discharged from the compressor under high pressure by means of the condenser 1 having the aforementioned structure, the coolant is fed into the header 2a from an inlet port (not shown) formed in part of the header 2a. During the course of flow through the plurality of flattened heat-transfer tubes 3, 3 from the header 2a to the header 2b, or during the course of travel between the headers 2a, 2b through the plurality of flattened heat-transfer tubes 3, 3 (where the headers 2a, 2b are respectively partitioned into small compartments), heat is exchanged between the heat-transfer tubes and a draft of air flowing from the front side to the rear side of the core 5, so that the coolant is condensed and liquefied.
A heat-transfer tube as illustrated in FIGS. 3 and 4 is used for part of the condenser 1 having the foregoing structure as one type of the flattened heat-transfer tubes 3, 3. This flattened heat-transfer tube 3 is made by the steps of folding one plate of aluminum alloy into an U-shaped form along its longitudinal center (which will become the folded portion 6); superimposing on each other plane portions 7, 7 formed at both ends of the aluminum alloy plate; and brazing the thus-superimposed plane portions 7, 7 to thereby form a joint 8. In order to effect efficient brazing of the plane portions 7, 7 as well as brazing of the flattened heat-transfer tubes 3, 3 with the corrugated fins 4, 4, there are used so-called clad materials in which brazing materials are formed on one side or both sides of a core material of the aluminum alloy plate. Further, in order to bond both ends of the flattened heat-transfer tubes 3, 3 in which the joints 8 (which extend from one longitudinal end of the flattened heat-transfer tubes 3, 3 in their cross sectional direction) to the headers 2a, 2b without clearance between them, through holes matched with the outer shapes of the flattened heat-transfer tubes 3, 3 are formed in the respective inner side surfaces of the headers 2a, 2b. A clearance between the outer circumferential surface of the ends of the respective flattened heat-transfer tubes 3, 3 and the inner circumferential edges of the through holes is filled with the brazing material which is laid on the surface of each of the flattened heat-transfer tubes 3, 3 and the aluminum alloy plate of the headers 2a, 2b. An inner fin 10 is provided in each of the flattened heat-transfer tubes 3, 3. This inner fin 10 contributes to improvements in this efficiency of heat exchange between a fluid circulating through each flattened heat-transfer tube 3 and the flattened heat-transfer tube 3, as well as to improvements in the resistance against the inner and outer pressure of each flattened heat-transfer tube 3, especially against the inner pressure produced inside of the flattened heat-transfer tube 3. Accordingly, the inner fin 10 and the inner circumferential surface of each flattened heat-transfer tube 3 are brazed together.
If the condenser 1 that includes the flattened heat-transfer tubes 3, 3 having the foregoing joints 8 is filled to an automobile, it is attached to the automobile while the joints 8 are positioned on the windward side (i.e., at a location on the left-hand side of FIG. 3). For example, FIG. 7 shows a whole view of an automobile in which the condenser 1 is installed at the front of the automobile. In FIG. 3, the draft of air flows from left to right as indicated by .alpha.. In general, the joints 8 are directed in the direction in which the automobile is headed. The reason for this is that even if foreign substances, such as pebbles, hit the front of the core 5 during the travel of the automobile, the joints 8 will receive the foreign substances and in so doing protect the main body of each of the flattened heat-transfer tubes 3, 3.
However, a recent study conducted by the inventors of this patent showed that there is a risk of damage to the durability of each of flattened heat-transfer tubes 3, 3 if the joints 8 are positioned on the windward side. Specifically, in order to test the durability of the condenser 1 having the flattened heat-transfer tubes 3, 3 as illustrated in FIGS. 3 and 4, the inventors performed tests in which steel balls having substantially the same weight as that of foreign substances (which have a high risk of hitting the front edges of the flattened heat-transfer tubes 3, 3) were brought into collision with the joints 8 at various angles. As a result of this test, if steel balls 8 come into collision with the joints 8 from the front at an angle, it has turned out that comparatively large stresses act on the curved areas 9, 9 carried from the joint 8 according to moment stress exerted on the joints 8. The stress exerted on the curved areas 9, 9 elastically deforms the curved areas 9, 9, as well as causing residual stress in each of the curved areas 9, 9. For example, FIG. 5 is a side view of the flattened heat-transfer tube 3 in a state that the foreign substance such as a pebble hits the plane portion 7 in a downward direction, so that the plane portions 7, 7 are directed upwardly. In this case, the stress easily concentrate and remain at the portion B. Consequently, there is a risk of loss of the durability of each of the flattened heat-transfer tubes 3, 3. Further, mud or the like are easily gathered at the portion B, which causes corrosion.